The Institute of Electrical and Electronic Engineers (IEEE) 802.16 Working Group on Broadband Wireless Access Standards is specifying standards for broadband radio communication systems in wireless metropolitan area networks. The IEEE 802.16 family of specifications is called the Wireless Metropolitan Area Network (WirelessMAN) standard and has been dubbed “WiMAX”, which is short for Worldwide Interoperability for Microwave Access, by an industry group called the WiMAX Forum. The mission of the WiMAX Forum is to promote and certify compatibility and interoperability of products complying with the IEEE 802.16 specifications.
The WirelessMAN standard defines aspects of the air interface between a radio transmitter and a radio receiver, including the physical (PHY) layer, and the Medium Access Control (MAC) layer. The WiMAX Forum has defined an architecture for connecting a WiMAX network with other networks, such as networks complying with IEEE 802.11 and cellular networks, and a variety of other aspects of operating a WiMAX network, including address allocation, authentication, etc. FIGS. 1A, 1B show examples of WiMAX networks, and it should be understood that the arrangement of functionalities depicted in FIGS. 1A, 1B can be modified in WiMAX and other communication systems.
As depicted in FIG. 1A, the network 100A includes base stations (BSs) 102, 104, 106, 108 that respectively transmit and receive radio signals in geographic areas called “cells”, which typically overlap to some extent as shown. Subscriber stations (SSs) 110, 112 are located in the cells and exchange radio signals with the BSs according to the WiMAX air interface standard. An SS is typically either a mobile SS (MS) or a fixed SS, and it will be understood that a network can include many cells and many SSs. In FIG. 1A, the BSs communicate with and are controlled by Access Service Network (ASN) Gateways (G/Ws) 114, 116 that also communicate with each other, and with other core network nodes and communication networks (not shown), such as the public switched telephone network and the internet. SSs, such as SSs 110, 112, can be organized into groups for paging, as described in more detail below.
FIG. 1B depicts a WiMAX network 100B that also includes BSs 102, 104, 106, 108 and SSs 110, 112 as in the network 100A. The network 100B is more decentralized than the network 100A in that, in FIG. 1B, the BSs communicate with each other directly through a suitable routing network 118 that also communicates with other core network nodes and communication networks (not shown).
According to one mode of IEEE 802.16, the downlink (DL) radio signals transmitted by the BSs are orthogonal frequency division multiple access (OFDMA) signals. In an OFDMA communication system, a data stream to be transmitted by a BS to a SS is portioned among a number of narrowband subcarriers, or tones, that are transmitted in parallel. Different groups of subcarriers can be used at different times for different SSs. Because each subcarrier is narrowband, each subcarrier experiences mainly flat fading, which makes it easier for a SS to demodulate each subcarrier.
The DL radio signals and uplink (UL) radio signals transmitted by the SSs are organized as successions of OFDMA frames, which are depicted in FIGS. 2A, 2B according to a time-division duplex (TDD) arrangement in the IEEE 802.16e standard. FIG. 2B is a magnification of FIG. 2A and shows the format of the DL and UL subframes in more detail than in FIG. 2A. In FIGS. 2A, 2B, time, i.e., OFDMA symbol number, is shown in the horizontal direction and subchannel logical number, i.e., OFDM subcarrier frequency, is indicated by the vertical direction. FIG. 2B shows one complete frame and a portion of a succeeding frame, with each DL subframe including sixteen symbols and each UL subframe including ten symbols, not counting guard symbols.
Each DL frame 200 starts with a preamble signal that includes a known binary signal sent on every third OFDM tone or subcarrier, as depicted by FIG. 3 in the frequency domain for a 2048-point fast Fourier transform (FFT). The range of subcarriers shown in FIG. 3 is numbered 0, 3, 6, . . . , 1701, but as explained below, a preamble can use fewer than that many subcarriers.
As seen in FIGS. 2A, 2B, each frame's preamble is followed by a DL transmission period and then an UL transmission period. According to the standard, the preamble signal is sent in the first OFDM symbol of a frame, which is identified by an index k in FIG. 2B, and is defined by the segment, i.e., one of the three sets of tones to be used, and a parameter IDCell, which is the transmitting cell's identification (ID) information. A SS uses the preamble for initial synchronization of its receiver to the BS (the network), and to determine the location of a frame control header (FCH), which is among the first bursts appearing in the DL portion of a frame. A SS also uses the preambles in signals transmitted by neighboring BSs to synchronize to them for purposes of measurement for handover from one cell to another.
The FCH gives information on the DL signal parameters, including a DL map message (DL-MAP), which is a medium access control (MAC) message that defines DL allocations for data, and parameters relevant for reception of the signal. The DL-MAP may be followed by an UL map message (UL-MAP), which provides UL allocations for data, and other parameters relevant for transmission of signals from an identified SS. With the assignments in time and frequency from the DL-MAP, an identified SS can receive the data in the particular location. Similarly, it can identify assignments in time and frequency on the UL-MAP, and transmit accordingly. FIGS. 2A, 2B also show a transmit/receive transition gap (TTG) interval and a receive/transmit transition gap (RTG) interval, which are used by the BS and SS to switch from transmit to receive and vice versa.
FIG. 2A also illustrates how a BS pages an SS operating in idle mode, showing the relationship between paging cycles, paging offset, BS paging interval, and OFDMA frames. Only two of the succession of paging cycles are shown in FIG. 2A. An SS “listens” for a page message from the BS during only a portion of a paging cycle, and the location of that paging interval is determined by a paging offset from the start of the paging cycle. A paging interval can span up to several (e.g., five) OFDMA frames, during which the SS needs to stay “awake” until its paging message received.
Thus, while a SS is idle, the SS periodically turns on its baseband unit, which includes a FFT demodulator and decoder, even when there are no paging messages for it and no system configuration changes/updates. The SS first synchronizes with the preamble and reads the FCH, and it then reads the DL-MAP to look for the location and the format of a broadcast connection identifier (CID). If the DL-MAP shows a broadcast CID, the SS demodulates that burst to determine whether there is a BS broadcast paging message (MOB_PAG-ADV).
Most of the time, there is no paging message and no action required by the SS, but during each paging interval, an SS has to be fully “awake”, which is to say, its receiver has to be powered up, for a number of OFDMA frames, using electrical power and possibly draining a battery over time. In addition to MOB_PAG-ADV messages, changes in channel descriptors or broadcast system updates can trigger an idle SS to stay on for updating the system parameters or reading other coming messages.
A “quick” paging mechanism that can reduce the negative effects of the conventional paging mechanism is not specified in current versions of the WiMAX standards. In such a quick paging mechanism, a simple signal would indicate to a group of SSs that a paging signal exists in a subsequently transmitted signal block. Thus far, proposals for quick paging either steal system resources from a system's available resources, thereby reducing system capacity, or occupy transmit and receive gaps in a TDD version of the system, which could lead to issues of compatibility among different device implementations.
A new standard for mobile broadband communication is under development as IEEE 802.16m, which is required to be backward-compatible with products complying with the current WiMAX standards and at the same time should improve performance considerably compared to current WiMAX technology. In developing IEEE 802.16m, a proposal has been made for a quick paging mechanism that is described in IEEE C802.16m07/217, “Wake-up Signal for 802.16m OFDMA Idle Mode” (Nov. 7, 2007). If an SS decodes the quick paging signal correctly, the SS needs to listen to the conventional paging signal; otherwise, the SS can go back to “sleep”, thereby saving its resources, such as battery power.
U.S. Provisional Patent Application No. 61/014,471 filed on Dec. 18, 2007, which is now U.S. patent application Ser. No. 12/808 779, filed on Jun. 17, 2010, by the current inventors describes using unused subcarriers (i.e., unused system resources) in a preamble signal to send assigned code words for quick paging. The code words assigned to SSs can include unused conventional preamble sequences and orthogonal sequences, such as Walsh-Hadamard (W-H) sequences, or bi-orthogonal sequences, such as W-H sequences and their inverses. Those patent applications are incorporated here by reference.
For one example, a W-H code word can be used as the signal for quick paging as described in the patent applications cited above. With a 10-MHz-wide WiMAX channel using an FFT of length 1024 bits, the length of the conventional preamble is 284 bits. Thus, there are 568 unused subcarrier positions that can be used for a quick paging signal, and so a W-H code word of length 512 bits can be used. For a 5-MHz-wide WiMAX channel, the FFT size is 512 bits and the preamble length is 143 bits, and so 286 unused subcarrier positions are available for the quick paging signal, thereby allowing use of a W-H code word of length 256 bits. Other channel bandwidths, such as 8.75 MHz, can be accommodated in a similar manner. Each such quick paging code word can identify a respective group of SSs, and the presence of a code word in a DL signal indicates to the SS(s) to which that code word is assigned that those SS(s) are required to read the full paging message in a subsequent DL signal.
It is known that W-H code words do not have particularly desirable spectral properties, and so a pseudorandom-noise (PN) masking sequence can be combined with a W-H code word, e.g., by a logical exclusive-OR operation. As described in the above-cited patent applications, the PN masking sequence can be chosen as a sub-sequence of a length-1023 PN sequence that can be generated using a shift register. Different cells in a network can use different shifts of the PN sequence.
In cellular telephone networks using code division multiple access (CDMA), such as CDMA2000 and wideband CDMA (WCDMA) networks, paging groups are predefined by the applicable standards based on mobile station IDs. Similarly, a mapping between quick paging messages and mobile station IDs is also predefined. The cellular telephone architecture is centralized, and so a central node passes registration information about a mobile station to multiple cells in a paging area. Thus, the mobile station can be reached in any cell belonging to the assigned paging area using a quick paging message. Additionally, the mobile station informs the network whenever it enters a new cell that belongs to a different paging area, triggering defined paging area updating procedures.
Nevertheless, there is no quick paging mechanism standardized in WiMAX communication systems at this time, and thus no consideration of receiving methods and devices for such quick paging signals.